The invention concerns a process for the preparation of trifluoroacetyl compounds, in particular trifluoracetyl chloride by the reaction of 1,1,1-trifluoro-2,2,2-trichloroethane with sulfur trioxide in the presence of mercury salts.
Such a process is known from U.S. Pat. No. 3,160,659. According to this reference, perfluoroacetyl chloride may be obtained by introducing fuming sulfuric acid or SO.sub.3, slowly into trifluorotrichlorethane, to which mercury sulfates have been added as catalysts, at the reflux temperature and subsequently maintaining the mixture at the boiling temperature for a longer period of time. After a few hours, the trifluoracetyl chloride begins to distill off. Several days are needed to complete the reaction.
The preparation of fluorine containing perhalogen carboxylic acids by solvolysis is also difficult. The solvolysis of perchlorinated aliphatic compounds containing one or more CCl.sub.3 groups is relatively simple. They may be converted by means of a moderately concentrated sulfuric acid to carboxylic acids having one or more carboxyl groups, such as described, for example, by H. Henecka in Houben, Weyl, "Methods of Organic Chemistry", Vol. 8, p. 427. If one or several fluorine atoms are present adjacently to the terminal CCl.sub.3 group, carboxylic acid is formed only when sulfuric acid containing free SO.sub.3 or SO.sub.3 are added. By varying the SO.sub.3 content of the sulfuric acid, the ratio of the amount of SO.sub.3 to the alkane halide, the reaction temperature or the reaction time, attempts were made to improve the process so that industrial applications would be possible. The conversion is effected in part in the absence and in part in the presence of catalysts, such as mercury sulfates (U.S. Pat. Nos. 2,396,076 and 3,102,139). However, the reaction rates of the processes, which may be only be carried out discontinuously, remain unsatisfactory. A further disadvantage of these processes is the frequently laborious recovery of the acid from the reaction mixture. The latter generally is diluted with water and the acid recovered by means of extensive and costly extraction processes. The acid often decomposes during processing, which again reduces the yield.
According to German Pat. No. 19 17 630, fluorine-containing perhalogenated carboxylic acid fluorides or chlorides are prepared from fluorochloroalkanes by reaction with SO.sub.3 in the presence of catalytic amounts of mercury sulfates. This process is suitable for continuous operation, wherein usable but not optimum space-time yields are obtained.